slightly better waterfall warning

documentation/docs/98-reference/.generated/client-warnings.md

@@ -45,7 +45,7 @@ TODO
 ### await_waterfall
 
 ```
-Detected an unnecessary async waterfall
+An async value (%location%) was not read immediately after it resolved. This often indicates an unnecessary waterfall, which can slow down your app.
 ```
 
 TODO

packages/svelte/messages/client-warnings/warnings.md

@@ -38,7 +38,7 @@ TODO
 
 ## await_waterfall
 
-> Detected an unnecessary async waterfall
+> An async value (%location%) was not read immediately after it resolved. This often indicates an unnecessary waterfall, which can slow down your app.
 
 TODO
 

packages/svelte/src/compiler/phases/3-transform/client/visitors/VariableDeclaration.js

@@ -1,7 +1,7 @@
 /** @import { CallExpression, Expression, Identifier, Literal, VariableDeclaration, VariableDeclarator } from 'estree' */
 /** @import { Binding } from '#compiler' */
 /** @import { ComponentClientTransformState, ComponentContext } from '../types' */
-import { dev } from '../../../../state.js';
+import { dev, is_ignored, locate_node } from '../../../../state.js';
 import { extract_paths } from '../../../../utils/ast.js';
 import * as b from '../../../../utils/builders.js';
 import * as assert from '../../../../utils/assert.js';
@@ -19,7 +19,7 @@ export function VariableDeclaration(node, context) {
 
 	if (context.state.analysis.runes) {
 		for (const declarator of node.declarations) {
-			const init = declarator.init;
+			const init = /** @type {Expression} */ (declarator.init);
 			const rune = get_rune(init, context.state.scope);
 
 			if (
@@ -164,10 +164,23 @@ export function VariableDeclaration(node, context) {
 
 				if (declarator.id.type === 'Identifier') {
 					if (is_async) {
+						const location = dev && is_ignored(init, 'await_waterfall') && locate_node(init);
+
 						declarations.push(
 							b.declarator(
 								declarator.id,
-								b.call(b.await(b.call('$.save', b.call('$.async_derived', b.thunk(value, true)))))
+								b.call(
+									b.await(
+										b.call(
+											'$.save',
+											b.call(
+												'$.async_derived',
+												b.thunk(value, true),
+												location ? b.literal(location) : undefined
+											)
+										)
+									)
+								)
 							)
 						);
 					} else {

packages/svelte/src/constants.js

@@ -39,6 +39,7 @@ export const NAMESPACE_MATHML = 'http://www.w3.org/1998/Math/MathML';
 // we use a list of ignorable runtime warnings because not every runtime warning
 // can be ignored and we want to keep the validation for svelte-ignore in place
 export const IGNORABLE_RUNTIME_WARNINGS = /** @type {const} */ ([
+	'await_waterfall',
 	'state_snapshot_uncloneable',
 	'binding_property_non_reactive',
 	'hydration_attribute_changed',

packages/svelte/src/internal/client/reactivity/deriveds.js

@@ -88,11 +88,11 @@ export function derived(fn) {
 /**
  * @template V
  * @param {() => Promise<V>} fn
- * @param {boolean} detect_waterfall Whether to print a warning if the value is not read immediately after update
+ * @param {string} [location] If provided, print a warning if the value is not read immediately after update
  * @returns {Promise<Source<V>>}
  */
 /*#__NO_SIDE_EFFECTS__*/
-export function async_derived(fn, detect_waterfall = true) {
+export function async_derived(fn, location) {
 	let parent = /** @type {Effect | null} */ (active_effect);
 
 	if (parent === null) {
@@ -129,12 +129,12 @@ export function async_derived(fn, detect_waterfall = true) {
 
 					internal_set(signal, v);
 
-					if (DEV && detect_waterfall) {
+					if (DEV && location !== undefined) {
 						recent_async_deriveds.add(signal);
 
 						setTimeout(() => {
 							if (recent_async_deriveds.has(signal)) {
-								w.await_waterfall();
+								w.await_waterfall(location);
 								recent_async_deriveds.delete(signal);
 							}
 						});

packages/svelte/src/internal/client/reactivity/effects.js

@@ -352,7 +352,7 @@ export function template_effect(fn, sync = [], async = [], d = derived) {
 		var restore = capture();
 		var unsuspend = suspend();
 
-		Promise.all(async.map((expression) => async_derived(expression, false))).then((result) => {
+		Promise.all(async.map((expression) => async_derived(expression))).then((result) => {
 			restore();
 
 			if ((effect.f & DESTROYED) !== 0) {

packages/svelte/src/internal/client/warnings.js

@@ -30,11 +30,12 @@ export function await_reactivity_loss() {
 }
 
 /**
- * Detected an unnecessary async waterfall
+ * An async value (%location%) was not read immediately after it resolved. This often indicates an unnecessary waterfall, which can slow down your app.
+ * @param {string} location
  */
-export function await_waterfall() {
+export function await_waterfall(location) {
 	if (DEV) {
-		console.warn(`%c[svelte] await_waterfall\n%cDetected an unnecessary async waterfall\nhttps://svelte.dev/e/await_waterfall`, bold, normal);
+		console.warn(`%c[svelte] await_waterfall\n%cAn async value (${location}) was not read immediately after it resolved. This often indicates an unnecessary waterfall, which can slow down your app.\nhttps://svelte.dev/e/await_waterfall`, bold, normal);
 	} else {
 		console.warn(`https://svelte.dev/e/await_waterfall`);
 	}